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Abstract:

Introduction. Tumor hypoxia and elevated glycolysis (Warburg effect) predicts poor prognosis. Each parameter is assessable separately with Positron Emission Tomography but they are linked through anaerobic glycolysis (Pasteur effect). Here we compare the oxygenation-dependent retention of fluoroazomycin arabinoside ([18F]FAZA), a promising but not-well characterised hypoxia-specific tracer, and fluorodeoxyglucose ([18F]FDG) in 4 carcinoma cell lines. Methods. Cells seeded on coverslips were positioned in modified Petri dishes that allow physically separated cells to share the same tracer-containing medium pool, thus, allowing simultaneous assessment of tracer retention in several cell lines under identical conditions. Following oxic ([O2]=20%), hypoxic ([O2]=1%) or anoxic ([O2] <0.02%) tracer incubation coverslips were analysed for radioactivity ([18F]FDG+[18F]FAZA) or re-incubated in tracer-free oxygenated medium and then measured (only [18F]FAZA). Next, we tested the reliability of [18F]FDG as a relative measure of glucose metabolic rate. In two cell lines tracer uptake was also quantified in vivo by assessing tracer retention in hypoxic and well-oxygenated xenograft-derived tissue sections. Results. Three hours of anoxia strongly stimulated [18F]FAZA retention with anoxic-to-oxic uptake ratios typically above 30. Subsequent transfer of tracer-loaded cells to tracer-free medium suggests that the true hypoxia-specificity is even higher as previously oxic, but not anoxic, incubated cells are largely cleared of tracer. Conversely, oxic uptake and anoxic-to-oxic uptake ratio of [18F]FDG varied considerably between cell lines and this inter-cell line variability reflects real difference in glucose consumption rather than differences in the selectivity of FDG versus glucose. Although less pronounced, [18F]FAZA also showed superior in vivo hypoxia-specificity compared with [18F]FDG. Conclusions. [18F]FAZA displays excellent in vitro characteristics with modest cell-to-cell line variability and no binding in oxic cells. In contrast, the usability of [18F]FDG as a surrogate marker for hypoxia is questionable due to large variations in baseline (oxic) uptake rates and variable magnitudes of the Pasteur effects. Supported by grants from the EU-funded Biocare project and the Danish Cancer Society